Lots actually. MMS is hard - multiple devices with no consistency, different interconnect gateways that can mangle messages along the way. An easy to use, reliable and pervasive MMS gateway isn't as easy to implement as some might think and the carriers haven't made it a priority. In fact the US carriers completely messed up their early MMS implementations with Washington-style regulation that made the user experience beyond horrible. Here's hoping Twilio makes some progress with this and I wish them much success.

EDIT: I realized that I didn't actually answer your question. MMS can be supported by carriers in different ways but the most typical interface is MM7 which is SOAP over HTTP. There are many MM7 gateway vendors and they all handle MIME boundaries, payloads, transcoding, etc different - subtle differences, but enough to make testing against a large matrix of devices very, very hard.